1. Field of the Invention
This invention relates generally to a method for determining or predicting the durability of a multi-layer composite structure, such as a multi-layer polymeric coating, and, more particularly, to a method for determining the effects of changes in composition and/or processing parameters on the durability of a composite structure.
2. Technical Considerations
Composite structures can take many forms. One example of a composite structure is a multi-layer polymeric coating. The durability of such composite coatings is an important feature to many consumers. By “durability” is meant the ability of the coating to withstand various weathering conditions, such as rain, humidity, sunlight, heat, cold, and various chemicals without failure of the coating. Coating “failure” can take the form of delamination of the coating from the substrate or from a previously applied coating layer on which it is deposited. Other types of coating failure could include blistering, loss of color, loss of adhesion, and various other changes in the mechanical and/or chemical properties of the coating that would make the coating less desirable for its intended use.
In an effort to improve or alter coating characteristics and/or to streamline production processes, coating producers may change the composition of a particular coating and/or one or more of the processing parameters under which the coating is formed. However, even minor changes in the coating composition and/or the processing parameters can impact upon the durability of the subsequent coating. In the past, the impact on coating durability of any such changes would be ascertained by subjecting the modified coating to a weathering atmosphere, such as an outdoor atmosphere, for an extended period of time to test the durability of the modified coating. This process could take many months, if not years, to complete to provide an accurate assessment of the coating durability.
As an alternative to this time consuming process, accelerated weathering procedures have been developed to attempt to predict coating durability in a shorter length of time. These accelerated procedures typically rely upon exposing the coating to artificial conditions, such as short wavelength energy, high intensity light, and high humidity, in order to accelerate coating degradation. The coating durability is then estimated based upon the results of exposure to these artificial conditions. However, many of the artificial tests still take months to complete. Additionally, coating durability estimates based on such artificial conditions may not correlate well to coating durability under actual use conditions.
Recently, methods for estimating coating durability which can be done in a shorter length of time than prior conventional tests have been developed. One such method is disclosed in European Patent Application EP 1,229,321. In this method, a set of coatings is exposed to the same ambient outdoor conditions for the same period of time. The exposed coatings are then subjected to a chemiluminescence test. The results of the chemiluminescence test for one selected coating are compared to the chemiluminescence results for at least one other coating of the set to estimate coating durability based on the coating chemiluminescence. However, this method cannot estimate a time until coating failure. Additionally, this method is based on measuring the light signal that arises from heat-induced decomposition of a species when the coating is exposed to outdoor conditions. Should such species not be present or should the type of failure attempted to be quantified not be based on the presence of such species, this method would not be applicable.
Therefore, it would be advantageous to provide a method of predicting the durability of a composite structure, such as a composite polymeric coating, that eliminates or reduces at least some of the problems currently encountered.